The distribution of the unpaired electron over the oxygen and the 24 carbon atoms in the free 2,4,6-triphenylphenoxy radical was determined by electron spin resonance spectroscopy andquantum-mechanical approximation methods. The hyperfine splitting was evaluated with the aid of the spectra of triphenylphenoxyls deuterated in some or all of the substituent phenyl groups. The results of the quantum-mechanical approximations were checked by recording the ESR spectra of triphenylphenoxyls labeled with 13C in positions I , 2,3, or 4 of the central ring. The spin density distribution permits a first discussion of the 1 7 0coupling constants of correspondingly labeled triphenylphenoxyl and other organic free radicals.
The dehydrogenation of 2,4,6-triphenylphenoI 111 and many other 2,4,6-triarylphenols[zl in an alkaline or a neutral medium leads to phenoxy radicals, which are unusually stable: they do not react with oxygen and, despite their high oxidation potentials 131, they can be kept in organic solvents for a long time without any change. As in the case of 2,4,6-tri-t-butylphenoxyl[41, blocking of the reactive 2, 4, and 6 positions with aryl groups suppresses the susceptibility of the ring to further addition and substitution. Since the steric hindrance with phenyl groups is less than with t-butyl groupsanother factor must be present in 2,4,6-triphenylphenoxyl to compensate for the decrease in steric hindrance. This effect is due to the participation of the phenyl groups in the 7c-electron system thus enabling the unpaired electron to occupy 25 positions, as compared to only seven in phenoxyl or trialkylphenoxyls.
The distribution of the unpaired electrons over the various atoms can be determined from the ESR spectra[sl, and in particular from the resulting coupling constants and empirical relations between the latter and the spin densities. The